Conventionally, as a type of a tractor, there has been known a tractor which includes, in a transmission casing thereof, a traveling-system power transmission mechanism which is interposed between an engine and an axle, a first PTO-system power transmission mechanism which is interposed between an engine and a PTO shaft, and a second PTO-system power transmission mechanism which is interposed between a downstream portion of the traveling-system power transmission mechanism and a PTO shaft (see Japanese Utility Model Accepted Publication Hei7(1995)-10925, for example).
Further, a PTO transmission chamber is formed in a rear portion of the inside of the transmission casing, and in the inside of the PTO transmission chamber, a PTO shaft, a PTO transmission mechanism which performs a speed change of the PTO shaft, and a change over mechanism which selectively connects either one of a downstream end portion of the first PTO-system power transmission mechanism and a downstream end portion of the second PTO-system power transmission mechanism with the PTO shaft in a changeable manner are provided.
Further, in the traveling-system power transmission mechanism, an axel is interlockingly connected with a distal end portion of a traveling-system power transmission shaft which has a proximal end portion thereof interlockingly connected with the engine by way of a differential mechanism, wherein the differential mechanism and the above-mentioned first PTO-system power transmission mechanism, second PTO-system power transmission mechanism and PTO transmission chamber are arranged in a rear portion in the inside of the transmission casing.
Here, the first PTO-system power transmission mechanism is a power transmission mechanism which transmits a rotational speed proportional to a rotational speed of the engine to the PTO shaft (so-called live PTO-system power transmission mechanism), while the second PTO-system power transmission mechanism is a power transmission mechanism which transmits a rotational speed proportional to a vehicle speed to the PTO shaft (a so-called ground PTO-system power transmission mechanism).
In the above-mentioned tractor, however, since the differential mechanism, the first PTO-system power transmission mechanism, the second PTO-system power transmission mechanism and the PTO transmission chamber are arranged in the rear portion of the inside of the transmission casing, the transmission casing becomes large sized, whereby a weight and a manufacturing cost of the transmission casing are increased and, at the same time, the structure of the transmission casing becomes complicated thus leading to a cumbersome maintenance operation or the like.
Further, in the inside of the PTO transmission chamber, besides the PTO transmission mechanism, the changeover mechanism which connects either one of the downstream end portion of the first PTO-system power transmission mechanism and the downstream end portion of the second PTO-system power transmission mechanism with the PTO shaft in a selectively changeable manner is provided and hence, the structure of the inside of the PTO transmission chamber becomes complicated thus making a maintenance operation of the inside of the PTO transmission chamber or the like cumbersome.